For certain applications it is necessary to determine automatically the position of a beam of light which is projected from one location onto a target at another location. Applications requiring such a device may include laser communication systems wherein the communication is by the modulation of a laser between two points, one or both of which may be moving; the alignment of mechanical structures or civil engineering works; and the measurement of the deviation of a borehole from a predetermined direction.
Without restricting the generality of the application of this invention, but with the objective of promoting the understanding of its function, the invention will be discussed in respect of the last application stated.
In borehole drilling, particularly for mineral exploration, holes of small diameter, e.g., 4.7 cm, but often of great length, e.g., 1000 m or more, may be drilled. Because of the flexibility of the long string of rods required for the drilling and the anisotropic mechanical properties of the rocks, the holes which are drilled tend to wander from their original orientation. Under unfavourable circumstances boreholes have been known to change direction by as much as 180.degree. over their length. Since the data obtained from such boreholes normally is used to determine the economic feasibility of mining a deposit, it is readily apparent that a proper knowledge of the true path of the borehole is vital.
A number of devices have been employed in the past to map the orientation of a borehole throughout its length, but all of these have important limitations. The simplest device is a glass test tube containing some hydrofluoric acid. The test tube is lowered to a position in the borehole and retrieved after enough time for the upper surface of the acid to etch a line on the test tube. The line gives the dip of the borehole at that position. This method is very time-consuming and gives no information about the azimuth of the borehole.
More elaborate devices employ a compass and a gravity operated device which are either automatically "fixed" at the desired depth and retrieved to give the dip and azimuth of the borehole or else photographed periodically by a small, in-hole camera. In the latter instance the film, when developed, will show a series of dip and azimuth combinations. The disadvantages of these devices are that they are slow, give relatively few data points in the borehole, and cannot be used in boreholes which are metal cased or which pass through magnetite or pyrrhotite-rich rocks.
A third type of device employs gyroscopes to give a continuous measurement of dip and azimuth of the borehole. These are better than the previously mentioned types because they can give continuity of measurement, but they are not satisfactory in the smallest diameter boreholes required in mining because of the unreliability of the very small gyros required (less than 2 cm in diameter).